1. Field of the Invention
The invention relates to a braking circuit for a door actuator with a generatively operable electrical motor. “Door actuator” includes both door closers and door operators, namely swing leaf drives, sliding door drives, revolving door drives, or the like, having an electrical motor which is generatively operated in at least one direction of movement of a connected door leaf.
2. Description of the Related Art
Door operators, in which an electrical motor is generatively operated for closing a connected door leaf, are known for example from the document DE 10 2004 059 843 B3.
Two braking circuits are known from the document DE 10 2005028 057 A1.
According to a first variant, diodes, coupled to each other in series, are utilized as load resistance, the forward directions of the diodes having one and the same current direction. Some diodes have switches coupled in parallel. When closing the respective current circuit by the associated switch, the diode coupled in parallel is shunted and thus its electrical (partial load) resistance is turned off. The braking effect or attenuation effect, generated by the diodes, can thereby be modified, increased in the present case. A disadvantage is that the braking effect, respectively the attenuation effect, can only be modified in stages. This may result in unwanted jerky movements of the connected door leaf. In addition, the diodes, with regard to their breakdown voltage, are dependent on temperature, a circumstance which is disadvantageous in outside installed door operators.
According to one embodiment, the braking circuit realized is by a field-effect transistor, the drain and source terminals thereof, in the generator mode of operation, coupled to corresponding connectors of the electrical motor. The gate terminal of the field-effect transistor is coupled to a center tap of a voltage divider, which is coupled in parallel to the drain-source section of the field-effect transistor. In the resistive branch of the voltage divider, coupled in parallel to the gate-source section of the field-effect transistor, the voltage divider has resistors which can be switched on or switched off by switches. Field-effect transistors are disadvantageous in that they present a relatively high minimum gate-source voltage, which limits their application range with low generator voltages.